The present invention relates generally to Kraft recovery boilers used in the pulp and paper industry and, in particular, to a fume sensor system that measures the concentration of fume particles produced during combustion of black liquor in such Kraft recovery boilers.
Chapter 26 of Steam/Its Generation and Use, 40th ed, Stultz and Kitto, Eds., Copyright (copyright)1992, The Babcock and Wilcox Company, describes the Kraft pulping process. In that process, wood chips are fed to a digester where they are cooked under pressure in a steam heated aqueous solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S) known as white liquor or cooking liquor. In the digester the lignin in the wood pulp is dissolved, the Na2S is converted to Na2SO4, and the NaOH is converted to Na2CO3. After cooking, the pulp is separated from the residual liquor in a process known as brown stock washing. Following washing, the pulp is screened and cleaned to remove knots and shives and to produce fiber for use in the final pulp and paper products.
The black liquor rinsed from the pulp in the washers is an aqueous solution containing wood lignin, organic material and inorganic compounds oxidized in the cooking process. The Kraft cycle processes this liquor through a series of operations, including evaporation, combustion of organic materials, reduction of the spent inorganic compounds, and reconstitution of the white liquor. The Kraft recovery boiler furnace was specially designed to combust the black liquor organic material while, reducing the oxidized inorganic material in a pile, or bed, supported by the furnace floor. The molten inorganic chemicals or smelt in the bed are discharged to a tank and dissolved to form green liquor. Green liquor active chemicals are Na2CO3 and Na2S.
The black liquor solution, which contains these sodium compounds and combustible lignin, leaves the digester along with the wood pulp. During black liquor combustion in the furnace of the Kraft recovery boiler, the residual water is evaporated and the organic material is combusted. Approximately 45% by weight of the dry, as-fired solids is inorganic ash, and the majority of these inorganics are removed from the furnace as Na2S and Na2CO3 in the molten smelt. A significant amount of the ash is present as particulate entrained in the existing flue gases; generally, about 8% by weight of the entering black liquor solids leaves the furnace as ash.
Ash is generally categorized as fume or carryover. Carryover consists of char particles and black liquor droplets that are swept away from the char bed and liquor spray by the upward flue gas flow. Fume consists of volatile sodium compounds and potassium compounds, and it is vaporous in the combustion zone such that it is entrained in the flue gas and rises into the convection sections of recovery boilers. Since these volatiles condense into submicron particles that deposit onto the superheater, boiler bank, and economizer surfaces, it is desirable to minimize the amount of fume produced. The rate of fume production depends on local temperature within and above the smelt bed as well as the temperature distribution on the surface of the smelt bed. Fume particles in Kraft recovery boilers are usually 0.25 to 1.0 xcexcm (microns) in diameter and consist primarily of Na2SO4 and a much lower content of Na2CO3. Fume also contains potassium and chloride salts.
The much larger carryover particles, typically 5 to 100 xcexcm, are easily distinguishable from the submicron fume particles on the basis of size. Fume and carryover ash are also different in their chemical analyses. Carryover is similar in composition to the smelt. Fume is mostly Na2SO4 and is enriched in potassium and chloride relative to their composition in the smelt.
Fume which exits the recovery boiler furnace and makes its way into the convection pass of the recovery boiler is a major source of deposits on the steam generator tubing located within the convection pass. The fume deposits are generally removed by sootblowing. At temperatures below 600xc2x0 F., the fume deposits slowly sinter on the tubes. At 900xc2x0 F., the fume deposits sinter quickly and may harden and become resistant to sootblowing within an hour. If there are large amounts of carryover particles in the flue gas, these carryover particles can impact and become embedded in the fume deposits on the tubing. To avoid plugging, it is desirable to minimize production of both fume particles and carryover particles and to maintain good control of the furnace exit gas temperature of the Kraft recovery boiler.
The capability to make continuous, real-time, in-situ measurements of fume concentration in a Kraft recovery boiler has many potential benefits, including the ability to: (a) confirm the proper smelt bed temperature profile; (b) warn of potential hot spots in the smelt bed; (c) provide an alarm when excessive fume concentrations exist in the convection pass so that the sootblower cleaning system can be activated; (d) track fume concentration in the convection pass to improve overall fume collection efficiency, reduce fume particulate emissions, and recover the maximum amount of Na2SO4 for return to the liquor cycle in the Kraft process; and (e) provide a control signal to automatically add new Na2SO4 to the process.
Accordingly, one aspect of the present invention is drawn to a fume sensor system for measuring a concentration of fume particles produced during combustion of black liquor in Kraft recovery boilers. The main components of the fume sensor system according to the invention comprise: a fume sensor probe housing for insertion into an upper furnace region of the Kraft recovery boiler; laser means for producing collimated light which is projected into the furnace flue gases to interrogate same, objective lens means for projecting the collimated light from the laser means into the flue gases and receiving backscattered light from fume particles in the flue gases; light detection means for detecting backscattered light collected by the objective lens means and producing electrical signals indicative thereof; optical fiber means for conveying light between the laser means, the objective lens means, and the light detection means; and signal processing means for processing the electrical signals representative of the received backscattered light to produce signals representative of fume particle concentration in the flue gas.
The present invention relies upon the fact that the backscattered light intensity from submicron size fume particles is independent of the particle size and particle size distribution. This aspect is illustrated in FIG. 6. While carryover particles are also present in the furnace flue gases, those carryover particles are much larger in size (typically 5 to 100 xcexcm in size) and less numerous than the fume particles (typically 0.25 to 1 xcexcm in size). Thus, these particles may be easily discriminated.
To insure that background light in the recovery boiler furnace does not interfere with fume particle concentration measurements which might otherwise saturate the light detection means, another aspect of the present invention is drawn to noise discrimination by providing a combination of modulating the signal of interest and filtering the background noise with a narrow bandpass optical filter to reduce the background light in the fume sensor and prevent detector saturation.
Yet another aspect of the present invention is drawn to a hybrid objective lens assembly for a fume sensor probe comprising a plano-convex lens (needed for light collection) which is provided with a cylindrical graded index lens (needed for light projection). The hybrid objective lens assembly projects a light beam into the furnace, collects backscattered light from the fume particles in the furnace flue gas, and focuses the light into an optical fiber. A separate fiber may be used to deliver the laser light to the fume sensor probe. An important feature of the present invention involves the use of spatial filtering to discriminate against backscattered light from non-representative particle concentrations near the recovery boiler furnace walls.